Mobile systems generally refer to various telecommunication systems that enable private wireless data transmission for subscribers moving within the system. A typical mobile system is a public land mobile network (PLMN). The PLMN comprises fixed radio stations (base stations) located in the service area of the mobile network, the radio coverage areas (cells) of the base stations providing a uniform cellular network. A base station provides in the cell a radio interface (air interface) for communication between a mobile station and the PLMN. Since mobile stations can move in the network and they have access to the PLMN through any base station, the PLMNs are provided with complicated arrangements for subscriber data management, authentication and location management of mobile subscribers, for handovers (a change of a base station during a call) etc. The networks are also provided with services that support the transmission of information other than the usual speech calls (speech service), such as data, facsimile, video image, etc. These new services have required a considerable amount of developmental work and new arrangements in the networks.
Another area of mobile systems includes satellite-based mobile services. In a satellite system, radio coverage is obtained with satellites instead of terrestrial base stations. The satellites are located on an orbit circling the earth and transmitting radio signals between mobile stations (or user terminals UT) and land earth stations (LES). The beam of the satellite provides on the earth a coverage area, i.e. a cell. The coverage areas of individual satellites are arranged to form continuous coverage so that a mobile station is located at all times within the coverage area of at least one satellite. The number of the satellites needed depends on the desired coverage. Continuous coverage on the surface of the earth might require for example 10 satellites.
Subscriber mobility requires similar arrangements in satellite mobile systems as in the PLMNs, i.e. subscriber data management, authentication and location management of mobile subscribers, handovers, etc. The satellite systems should also support similar services as the PLMNs.
One way of implementing these requirements in satellite mobile systems is to use existing PLMN arrangements. In principle this alternative is very simple since a satellite system can be basically compared to a base station system of a mobile system having an incompatible radio interface. In other words, it is possible to use a conventional PLMN infrastructure where the base station system is a satellite system. In such a case, the same network infrastructure could in principle even contain both conventional PLMN base station systems and satellite "base station systems".
There are many practical problems related to the adaptation of the PLMN infrastructure and a satellite system, however. A problem apparent to the Applicant is that a PLMN traffic channel and a traffic channel of a "radio interface" in a satellite system differ considerably. Examine an example where the PLMN is the Pan-European digital mobile system GSM (Global System for Mobile Communication) and the satellite mobile system is the Inmarsat-P system that is currently under development.
A traffic channel in the GSM system supports data transmission at the user rates of 2400, 4800, 7200 and 9600 bit/s. In the future, high-speed data services (HSCSD=High speed circuit switched data) employing two or more traffic channels at the radio interface (multi-slot access) also support higher user rates (14400 bit/s, 19600 bit/s, . . . ). Non-transparent data services also utilize a radio link protocol RLP between a mobile station MS and an interworking function IWF, which is typically situated at a mobile services switching centre MSC. The RLP is a frame-structured balanced (HDLC-type) data transmission protocol. Error correction in the RLP is based on the retransmissions of frames corrupted on the traffic channel, requested by the receiving party. The traffic channel employs channel coding that aims at decreasing the effect of transmission errors. Due to the channel coding and the other overhead information the bit rate at the radio interface will be higher than the actual user rate. The radio interface rates for the user rates of 2400, 4800 and 9600 bit/s are 3600, 6000 and 12000 bit/s, respectively.
The Inmarsat-P satellite system requires that standard data rates up to 4800 bit/s can be transmitted on one traffic channel (e.g. 1200, 2400, 4800 bit/s) and that standard data rates exceeding 4800 bit/s (e.g. 9600, 14400, 19200 bit/s, etc.) can be transmitted by using several parallel traffic channels, such as in the HSCSD service of the GSM system.
In the Inmarsat-P satellite system, the data rate of one traffic channel at the radio interface is at most 4800 bit/s, which equals the user data rate of 4800 bit/s at the terminal interface. In a data service employing two traffic channels the data rate at the radio interface equals the user data rate of 9600 bit/s at the terminal interface. In other words, an end-to-end traffic channel between an MS and an MSC is non-uniform since the capacity of the traffic channel section over the satellite leg is lower than that of the traffic channel section between an LES and the MSC. This non-uniformity of the traffic channel causes the following problems in non-transparent data services employing the RLP protocol.
Firstly, the MSC-IWF transmits data towards the MS at the same rate as data is received from the fixed network, such as the ISDN or the PSTN. In practice, this may signify a data rate of 12000 bit/s in a non-transparent call since the data modem of the IWF may operate in an autobauding mode towards the fixed network. The LES may transmit data that it receives from the MSC-IWF towards the MS at a considerably lower rate, i.e. 4800 bit/s. The MS may easily receive the data that the LES transmits with the lower data rate, but the data begins to accumulate and may be lost at the LES. However, the MSC-IWF continues transmitting at the full rate of 12000 bit/s until a transmission window set in the RLP protocol is full. A transmission window refers to the number of the RLP frames the transmitting party may transmit without receiving acknowledgment from the receiving party.
A similar problem may also occur when other types of radio interfaces are connected to PLMNs, for example wireless telephone systems where the capacity of the traffic channel section at the radio interface is lower than that of the traffic channel section in the remaining part of the PLMN.